1. Field of the Invention
The present invention relates to printed circuit boards (PCBs), and particularly to a PCB which can reduce common-mode noise and electro-magnetic radiation between two differential traces of a differential pair.
2. Description of Related Art
Referring to FIG. 2, a partial cross-sectional view of a conventional PCB is illustrated. As shown, the PCB 1 includes a differential pair 100 having two differential traces 101 and 102 disposed within a signal layer in an edge-coupled mode, and two reference layers 110 and 120 arranged at two sides of the signal layer. The differential impedance between the two differential traces 101 and 102 can be adjusted by changing the distance between the signal layer and the reference layers 110 and 120. A preferred value of the differential impedance is approximately 100 ohms.
However, some PCBs are very thin (such as a Flexible Printed Circuit Board (FPCB)) and the distance between the signal layer and the reference layers is difficult to adjust. A conventional means is cutting the reference layers into grids, but that can cause too much common-mode noise and electro-magnetic radiation.
What is desired, therefore, is to provide a PCB which overcomes the above problems.